Leon R. Kass, the University of Chicago medical ethicist who four years ago today was named by President Bush to head the newly created President's Council on Bioethics, will step down as chairman Oct. 1, the White House announced late Wednesday.

Kass, who led the 18-member group of philosophers, scientists, theologians and legal scholars as it plumbed the turbulent debates over human cloning, embryonic stem cell research, the creation of animal-human hybrids and other topics raised by rapid advances in biotechnology, asked to be relieved of the chairmanship, council spokeswoman Diane Gianelli said.

"He loved the job" and will continue to serve as a member of the council, Gianelli said, but he had been feeling increasingly burdened by the amount of work involved in being chairman.

The White House said it had selected as the new chairman Edmund Pellegrino, 85, a professor emeritus of medicine and medical ethics at Georgetown University Medical Center and a former president of Catholic University. He will join the council in October.

Both Kass and Pellegrino, a widely renowned Catholic medical ethicist, did not respond to inquiries yesterday.

Kass spent yesterday overseeing completion of the council's seventh major report, on the ethics of care-giving for the aged. In a brief written statement, he said he was "deeply grateful to President Bush for the privilege of chairing his bioethics council during these challenging times."

Kass often said he hoped to inspire ordinary people to think more deeply about the crossroads of technology and ethics. In one such effort, the council published an anthology of excerpts from popular literature, including works by Leo Tolstoy, William Shakespeare and Homer, that raised difficult bioethics questions.

Although widely respected for his intellect, Kass's history of opposition to some reproductive technologies and his general wariness of other biomedical trends, such as efforts to forestall aging, made him a thorn in the side of many researchers and liberal thinkers. In February 2004, he came under intense fire for his role in the dismissal of two council members with liberal views on embryonic stem cell research.

Kass repeatedly denied that the ejections were politically motivated, but the image was difficult to shake, especially given his occasional open involvement in political frays.

"I think Leon went too far to engage himself in the politics of the topics the council considered, writing newspaper op-eds and going on the think-tank circuit," said Arthur Caplan, chairman of the department of medical ethics at the University of Pennsylvania Medical School. In several cases, Caplan said, Kass seemed to be pushing for a consensus that would be in line with the White House's preordained views on a topic.

"I think that may have damaged some of what he tried to accomplish," Caplan said.

Others, however, praised Kass for his leadership, as demonstrated in part by the large number of papers the group published on tough issues.

"It's been a very productive council, and I think that's largely attributable to Leon," said O. Carter Snead, an associate professor of law at the University of Notre Dame who served for three years as the council's general counsel.

As for politics, Snead said, "It was totally inconsistent with [Kass's] vision to be a post-hoc think tank to help the White House justify its policies."

Pellegrino, who is also affiliated with Georgetown's Kennedy Institute of Ethics, will introduce himself to the council at its meeting today and will take the reins at the next meeting, as yet unscheduled. The group will decide what topic to address next, Gianelli said.